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# Optical Properties of Paper and Board

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### Optical Properties of Paper and Board

1. 1. Pele Oy Optical Properties in Papermaking March 2016 Pekka.Komulainen@clarinet.fi
2. 2. Pele Oy Paper under light  For optical properties it is important that paper is porous (like snow).  Paper under light shows the following main phenomena and paper properties: • Specular reflection gloss and smoothness • Scattered reflection brightness and opacity • Scattered absorption color and opacity • Refraction opacity • Transmission opacity 2 Snow - white Ice – ”glassy” Reflection Refraction Absorption Transmission Incident light
3. 3. Pele Oy 3 Light reflection from printed image  For optical properties most important is how air is distributed in the paper and for strength how fibers are distributed.  For good multicolor pictures it is important that paper reflects all wavelengths i.e. is white. Picture: Evans, DuPont
4. 4. Pele Oy Kubelka-Munk theory and light scattering  Paper is rough and porous material. This means that it reflects scattered light from the surface but also deeper from the paper.  Light scattering coefficient is a material property, which tells how much from the incoming light the material can reflect as scattered light.  Light scattering coefficient can be calculated from paper reflection measurements.  Refraction index of main paper components are very similar. Every surface between paper and air refracts light and scattering is better when there are more these surfaces.  The higher the paper density is the less there is light scattering. 4
5. 5. Pele Oy Absorption coefficient  Absorption coefficient can be calculated from the measured values of R∞ and R0. It depends on the special chemical groups in molecules. Typically these groups are present in all kind of dirt.  Lignin in a pulp has molecules which reflect only yellow light (=absorbs blue light) and thus reduce brightness very much.  Pulp bleaching is basically additional removal of lignin from the pulp. 5
6. 6. Pele Oy Reflection and transmission  R∞ is the reflection coefficient of so thick sheet pile that no light goes through. It correlates with brightness measurements.  From the formula one can see that scattering coefficient must be high and absorption coefficient low to get high brightness.  Several specialty papers require some or high transparency. These can be called glassine papers. 6
7. 7. Pele Oy Reflection factor of a sheet against a black backing Reflection factor of a large stack of sheets 7 Opacity definition ISO Opacity = Reflection factor of a sheet against a black backing Reflection factor of a sheet against a standard backing Tappi Opacity = 100 % Opacity Playing cards with black core
8. 8. Pele Oy Opacity measurement 8  In the paper industry, there are two different standards for opacity measurement. ISO 2471 (Printing Opacity) and TAPPI T425 (Contrast Ratio) are the relevant standards.  ISO 2471 utilizes the diffuse illumination and 0° viewing geometry (d/0°) which is the same as the ISO brightness geometry.  TAPPI T425 uses 15° illumination and diffuse viewing(15°/d) originally developed by Bausch & Lomb in the 1930's.  The illuminants of ISO and TAPPI are different where effective wavelengths are 557 and 572 nm. Also the measured ISO and TAPPI opacities are different. ISO 2471 TAPPI T425 Geometry d/0° 15°/d White Backing R∞ R0.89 Wavelength, nm 557 572
9. 9. Pele Oy Opacity and brightness  Opacity % is 100*R0/R∞. This simple formula tells that opacity is lower when brightness is higher.  It is very difficult to get high opacity when brightness is high. This is the reason that newsprint grammage can be about 45 gsm but copy paper must be about 80 gsm.  Opacity relates to the show-through of the printed image from the opposite side of the sheet, or the sheet under it. 9
10. 10. Pele Oy 10 Standard illuminants A = 2856 ºK, “electric lamp” B = 4874 ºK C = 6504 ºK, ”indoor daylight” D65 = 6774 ºK, ”outdoor daylight”D65 = more UV  D65 is outdoor daylight, where UV is fully included. C is about indoor daylight (less UV).  Illuminants C and D65 are used for paper measurements. The big difference is that D65 includes more UV light (wavelengths less than 400 nm).  Optical brighteners (OBA = FWA) convert this UV light to visible blue light thus increasing brightness values.
11. 11. Pele Oy Brightness measurements  There are two basic types of brightness measurements: directional and diffuse.  Directional brightness (TAPPI brightness – TAPPI 452) employs the 45º/0º geometry of the original GE-Photovolt instrument. It has been the standard in the U.S. and Japan. Fiber orientation has effect on the TAPPI brightness measurement.  Diffuse brightness employs a D/0º geometry where D indicates diffuse illumination from a sphere, making it insensitive to sample orientation. Diffuse brightness is the standard in much of the rest of the world.  Two types of diffuse brightness are commonly discussed, and a standard exists for both of them.  ISO C Brightness (ISO 2470-1, TAPPI 525) uses Illuminant C level of UV energy. It simulates normal office lighting conditions.  ISO D65 Brightness uses Illuminant D65 (daylight) according to ISO 2470-2. 11 Sample Photodetector Light source Sample Photodetector Light source Directional Diffuse
12. 12. Pele Oy 12 D65 illuminant and measuring slot of brightness  For brightness measurement a narrow wavelength ”slot” has been standardised. The dominant wavelength of this slot is 457 nm (blue) and the range is ± 44 nm. If paper is yellow it reflects less blue light. This measure is effective for bleaching (less lignin less yellowish).  D65 illuminant includes lot of UV light, which can be converted to blue light by using optical brighteners. This increases brightness.  When measuring and calculating whiteness all wavelength have effect and blue dyes increase whiteness. 0 20 40 60 80 100 120 350 400 450 500 550 600 650 700 Brightness is measured from the slot which is under the white curve.
13. 13. Pele Oy Brightness weighting function  When pulp is bleached the reflectance of all wavelengths increases. However, the increase is largest at blue end of the spectrum.  Pulp brightness measurement is not only the measurement of total reflection but especially the increase of blue reflection (or decrease of yellow lignin absorption). 13
14. 14. Pele Oy 14 UV-light and copy paper reflection spectrum  Brightness is measured with dominant wavelength of 457 nm. Illuminants C and especially D65 give higher brightness when optical brighteners are used. Actually reflection is lower close to 400 nm, where absorption is higher and the energy is moved to higher wavelengths. D65C No UV-light Picture: Nils Pauler 457
15. 15. Pele Oy Whiteness  Whiteness is the ratio of Red, Green and Blue reflectance. It is an attribute of a diffusing surface which denotes its similarity in color to preferred or standard white.  Measurable properties CIE Whiteness (ISO 11475) and tint equations can be stated as follows. • W = 2.41L* – 4.45b*(1–0.009(L*–96))–141.4 • T = –1.58a*–0.38b* Wavelength, nm Brightness Whiteness 15
16. 16. Pele Oy 16 Brightness and whiteness of copy papers  D65 brightness of European copy papers can be more than 100% with D65 light and optical brightening agents.  Whiteness is about 50 %-unit higher than conventional ISO-brightness and D65 brightness about 10 %-unit higher than ISO brightness with C illuminant. 60 70 80 90 100 110 120 130 140 150 160 1 2 3 4 5 6 7 8 9 10 11 Paper suppliers % Brightness, C/2º, ISO 2470:1999 Brightness, D65/10º, SCAN P-66 Whiteness, D65/10º, ISO 11475:1999 Recycled base
17. 17. Pele Oy Fluorescent whitening agent (FWA or OBA) 17  Fluorescent Whitening Agent (FWA or OBA) is used to increase the white appearance of papers by absorbing invisible ultraviolet light and re-emitting it in the blue region of the visible spectrum. It is widely used in Europe to make bright surface.  This strategy can compensate for a yellow tint of many types of pulps that have been bleached to moderate levels.
18. 18. Pele Oy Fluorescent component of brightness  Fluorescent component is the additional brightness obtained from the use of optical brighteners. It is determined by measuring the sample with and then without the effect of UV energy on the sample. 18 Picture: Technidyne
19. 19. Pele Oy Brightness comparisons  When comparing brightness it is important to know what method and light has been used. The following data is an example of differences. Data: http://www.crableengineering.com/presentations/ Technidyne D65 ISO TAPPI CIE D65 CIE C ISO-TAPPI D65 -ISO A 98,9 91,8 89,9 126,6 106,0 1,9 7,1 B 95,3 89,0 87,3 117,5 98,6 1,7 6,4 C 105,6 95,1 91,8 139,3 112,3 3,3 10,5 D 111,2 99,8 96,5 161,9 134,5 3,3 11,4 E 110,6 98,9 95,5 162,0 133,9 3,4 11,7 F 112,3 100,4 96,0 149,7 122,2 4,4 11,8 G 113,5 100,9 96,4 160,5 132,2 4,5 12,6 H 110,1 98,7 94,7 145,3 117,4 4,0 11,4 I 112,3 100,2 95,9 161,2 133,6 4,3 12,1 J 93,0 88,6 90,0 110,8 98,9 -1,4 4,4 K 102,0 92,5 90,6 130,9 106,5 1,9 9,5 L 110,1 99,1 95,0 152,5 126,3 4,1 11,1 M 110,1 99,5 96,1 150,6 124,5 3,4 10,7 N 107,2 96,7 94,1 153,6 127,9 2,6 10,6 O 106,9 96,3 93,0 143,0 115,7 3,3 10,6 P 110,3 99,2 96,1 146,3 119,0 3,1 11,1 Q 109,6 99,0 96,1 143,5 118,1 2,9 10,5 R 113,9 101,5 97,4 149,9 121,8 4,1 12,4 S 111,6 99,3 95,0 163,0 134,6 4,3 12,3 Brightness Elrepho Whiteness Elrepho Sample Delta Brightness 19
20. 20. Pele Oy Paper gloss metering principle  Tappi T 480 defines the specular gloss of paper and paperboard at 75 degrees (15 ° from the plane of paper). This method is suitable for low to moderate-gloss coated and uncoated papers as well as for most ink films on paper or paperboard.  The standard describes the technical requirements for a corresponding gloss meter.  Gloss measurements are normally made using standard equipment like Hunter, Lehman and Zehntner and giving a single mean gloss value. But, a sample can have a high mean gloss value (normally considered as good) but at the same time have a high gloss variation which is disturbing when looking at a printed picture. The micro gloss method can quantify such disturbing gloss textures and ‘glare effects’ on printed and unprinted surfaces. 20
21. 21. Pele Oy Example of gloss instrument  It must be remembered that paper gloss is different in different directions depending on fiber orientation. Machine direction gloss is highest and cross machine gloss lowest.  The Technidyne PROFILE/Plus Gloss automatically measures the gloss at 75º in the MD and CD according to the following Industry Standards: TAPPI Method T 480, ISO 8254-1. 21
22. 22. Pele Oy Effect of Raw Materials
23. 23. Pele Oy Uncoated paper raw materials  Mechanical printings and woodree uncoated papers. 23 Material Mech. % WF % Comment Fibers 60 – 100 70 - 100 Wood or non-wood fibres Fillers 40 – 0 30 - 0 Mineral or synthetic pigments Surface size - 0 - 5 Starch, CMC, PVA, synthetic size, optical brighteners etc. Functional chemicals 0 – 1 0 – 2 Internal sizes, dyes etc. (effect on paper properties) Performance chemicals for process <1 <1 Retention aids, defoamers, biocides etc. (effect on process performance) Water 5 – 10 4 – 7 To be in balance with air humidity
24. 24. Pele Oy 24 Fillers and coatings in papers  To improve optical properties of paper mineral pigments are used in papermaking. They can be added as a filler before headbox or to the surface as a coating with binders. Paper Grades Filler Pigment % Surface size per side g/m2 Coating per side g/m2 Woodcontaining Newsprint, TMP/GW Newsprint, DIP 0 - 5 5 - 15 0 0 - 1.5 0 0 Unctd Mechanical, TD, Bulky SC 5 - 15 15 - 35 0 0 0 - 5 0 Ctd Mechanical, LWC MWC, HWC 5 - 15 8 - 18 0 0 - 2 5 - 15 20 - 40 Woodfree Uncoated Woodfree, Copy Printing 15 - 30 10 - 25 1 - 2 1 - 2 0 0 - 5 Coated Woodfree, standard Premium Art 10 - 15 12 - 18 0 - 2 0 - 2 10 - 15 20 - 35
25. 25. Pele Oy Light scattering coefficient of pulps  Finer fibers and less bonding give better light scattering (opacity & brightness)  When mechanical pulps have lower freeness light scattering is better (more refining, more unbonded fines)  When chemical pulps have lower freeness light scattering is lower (more bonding, practically no unbonded fines)  Harwood has better scattering than softwood  The more chemicals in pulping is used the lower light scattering will be 25
26. 26. Pele Oy Brightness and chromophores  Brightness is not increasing linearly in bleaching. To make very bright pulp requires too much effort and bleaching chemicals.  It is good to know in papermaking that very small amounts of lower brightness components, such as mechanical pulp, low brightness clay or dirty process water reduces brightness very fast. 26
27. 27. Pele Oy Fillers and Coating Pigments
28. 28. Pele Oy Main pigment requirements High light scattering Carbonate, PCC Colour printing Low basis weight High brightness Good opacity 28
29. 29. Pele Oy 29 Effects of filler addition Positive effects  Water removal   Better formation  Drying shrinkage  dimensional stability   Brightness, opacity and color   Ink absorption more uniform  Smoothness and gloss   Costs  and printability  Negative effects  All general strength properties  Surface strength , dusting   Internal bond strength   Stiffness   Carbonate requires > 7 pH  Runnability   Retention , two-sidedness  Wire, felt and machine wear  With PCC bulk and porosity can increase, with other fillers they decrease
30. 30. Pele Oy 30 Fillers - agglomerated or dispersed?  Fillers should be first fixed to the fibres when they are dispersed. This would guarantee good retention, strength and optical effect (brightness and opacity) at the same time. Only fibres, good strength, Low opacity Fillers well dispersed  low retention, low strength, good opacity Fillers agglomerated  good retention and strength, low opacity Picture: E.Gruber
31. 31. Pele Oy 31 Filler distribution  Filler can either fill paper pores (left) or distribute evenly on fibre surfaces (right). Even distribution has good optical effect but reduces strength more. Good filler distributionBad filler distribution Pictures: Robert A Gill
32. 32. Pele Oy Main raw material requirements  High light scattering improves opacity and brightness. High absorption coefficient improves opacity but decreases brightness.  Fillers and pigments are good raw materials for optical properties (low k and high s). High brightness Good opacity High light scattering High light absorption Low light absorption Property s k High brightness + – High opacity + + 32
33. 33. Pele Oy 33 Total mineral content of paper & board Source: Omya
34. 34. Pele Oy Main fillers and coating pigments  Titanium dioxide is a special filler with high refractive index. It is the only possibility to improve opacity of impregnated and waxed papers. This is the reason that décor paper includes titanium dioxide.  When making opacity and brightness TiO2 is a very expensive filler or coating pigment. It is used in U.S. for this purpose but not in Europe where brightness and opacity are made with less expensive means.  Titanium dioxide price is more than three times pulp price while carbonate filler price can be less than half of pulp price. 34 Mineral Avg size Refractive Scattering Brightness Density Hardness Pigment Index coefficient µm cm 2 /g % kg/m 3 Mohs Clay 0,2-2,0 1,55-1,57 1100-1200 80-92 2580 2-2,5 Calcined clay 0,7-1,5 1,60 2600-3000 90-95 2600 4,0-5,0 GCC 0,7-3,0 1,5-1,7 1400-1700 85-95 2710-2930 3,0-4,0 PCC 0,3-3,0 1,5-1,7 2200-6000 96-100 2710-3830 3,0 TiO2 Anatase 0,2-0,4 2,5-2,55 4500-6000 98-100 3820-3970 5,5-6,0 TiO2 Rutile 0,2-0,4 2,6-2,9 4500-6000 98-100 4230-5500 6,0-7,0
35. 35. Pele Oy 35 Effect of filler content on tensile strength  Dry tensile strength is reduced about 50% when a normal 20% loading is used. Initial wet strength reduces even more.  Higher particle size gives better strength but optical effect will be lower due to lower light scattering. Täyteainepitoisuus, % Picture: Robert A Gill Filler content, % Tensile,km Particle size increases
36. 36. Pele Oy Critical properties of titanium dioxide 36 Anatase, RI = 2.5 Rutile, RI = 2.7 (more effective)  Several properties of TiO2 are different compared to other fillers. However, if the effects are compared at the same level of opacity increase, the detrimental effects with TiO2 are lower than with several other fillers and pigments.  Even if the price of TiO2 can be up to ten times compared to lowest price fillers, the cost can be lower because the usage can be only 10% of the use of main filler (1-2% of paper for printing papers).  Because the share of TiO2 as a filler is low a good retention is very important.  TiO2 absorbs UV light and the effect of OBA is reduced with TiO2. OBA is more used in Europe and TiO2 in North America.
37. 37. Pele Oy 37 Minerals in papermaking  Minerals are a fast growing raw materials of papermaking. Total amount of minerals in paper and board is globally over 10%. We are back in stone age.  Especially consumption of carbonates has been growing fast because they are white, easy to find everywhere and less expensive than fibers or clay. Stone Forest in Kunming, China
38. 38. Pele Oy Paper Color and Printing Illuminant Eye and brain
39. 39. Pele Oy Color mixing  When lights are mixed it is additive mixing (we add energy). When inks and paints are mixed it is subtractive mixing (we add absorption and reduce energy from reflection).  Pure red and green light produce yellow, red and blue make magenta, blue and green combine to make cyan, and all three together, when mixed at full intensity, create white.  For mixing of dye pigments, it is better to use the secondary colors, since they mix subtractively instead of additively. Using Cyan, Yellow and Magenta toners we can create colors on paper. 39 Red BlueGreen Yellow MagentaCyan
40. 40. Pele Oy Color gamut  In digital printing, when we talk about “gamut” we mean the color space of a device.  Devices can be divided into two categories: • RGB devices like scanners, monitors, digital cameras etc. • CMYK devices like laser printers, offset etc.  Normally, a CMYK printer device color space will be smaller (less saturated and fewer colors) than a RGB capture device color space. A typical RGB color space A typical CMYK color space 40
41. 41. Pele Oy RGB color system  Light with a wavelength between 600 and 700 nm is known as red light.  Light with a wavelength between 500 and 600 nm is known as green light.  Light with a wavelength between 400 and 500 nm is known as blue light.  By combining Red, Green and Blue light we can create all the colors of the visible light-spectrum. 400 500 600 700 nm Blue Green Red 41 Blue Green Red Cyan 1 1 0 Magenta 1 0 1 Yellow 0 1 1 1 = Reflection 0 = Absorption
42. 42. Pele Oy White paper reflection  In theory, white paper reflects all colors.  This is a theoretical statement, because different brands of paper have a different color.  This is why in color management it is very important to know what paper we are using.  This is not only true for the output, but also for the original. 42
43. 43. Pele Oy Yellow toner absorbs blue light  Yellow is the complement of blue.  Yellow toner absorbs blue light and reflects green and red light.  The reflected “G” and “R” light are seen as yellow. R G B Y C M 43
44. 44. Pele Oy Magenta toner absorbs green light  Magenta is the complement of green.  Magenta toner absorbs green light and reflects blue and red light.  The reflected “B” and “R” light are seen as magenta. 44 R G B Y C M
45. 45. Pele Oy Cyan toner absorbs red light  Cyan is the complement of red.  Cyan toner absorbs red light and reflects green and blue light.  The reflected “B” and “G” light are seen as cyan. R G B Y C M 45
46. 46. Pele Oy Mixing subtractive colors  Equal amounts of magenta and yellow toner produces red.  Equal amounts of cyan and yellow toner produces green.  Equal amounts of magenta and cyan toner produces blue. 46 Y C M
47. 47. Pele Oy Process black  In theory, equal amounts of C, M and Y produce black.  This black is called “Process Black”.  In reality, it is virtually impossible to produce true black using cyan, magenta and yellow toner. Depending on the used toners or inks, the result can vary form deep blue to be brown or gray. 47 Pure black Process black
48. 48. Pele Oy Yellow school bus  Yellow surface absorbs mainly blue rays. 48 Picture: HunterLab
49. 49. Pele Oy The CIE Lab model  CIE Lab is the second of two systems adopted by the CIE. It is an attempt to reduce the distortion in color distances.  Lab is based on XYZ, but is non- linear, to try to mimic the human senses. • L is a luminance scale. • a and b are color axes.  Although not perfect, it is the most useful system today. 49
50. 50. Pele Oy 50 Lab colour system  Vertical axis L is black and white axis.  b is yellow-blue axis. Bright paper has negative b values i.e. paper is bluish.  a is red-green axis. Bright paper has positive a values i.e. paper is reddish.  Blue-red paper looks brighter than yellow-green.
51. 51. Pele Oy 51 Multicolor process CMYK + + + = = =
52. 52. Pele Oy Some Practical Issues
53. 53. Pele Oy Papermaking variables and paper properties 53 Source: Michael Evans
54. 54. Pele Oy 54 Colour of white papers Kuva: Jouni Marttila L*a*b* L  Close to the neutral point all papers look white. When looking more closely, higher quality papers are bluish (-b values) and lower quality papers are yellowish (+b values).
55. 55. Pele Oy Example of commercial papers Brightness ISO (%) in relation to Opacity 90,0 91,0 92,0 93,0 94,0 95,0 96,0 97,0 98,0 65,0 65,5 66,0 66,5 67,0 67,5 68,0 68,5 69,0 69,5 70,0 70,5 71,0 71,5 72,0 SC-ASC-B NorCal 2011 Development of NorCal in relation to the competition > Measurements resulted from print trials. UPM ECO Stora Enso Publipress matt Stora Enso Envi Press SCA Grapho verde MY Joy Stora Enso maxau SCA Grapho verde UPM ECO Brightness C2 (%) MY Joy Blue Opacity (%) 55
56. 56. Pele Oy AF&PA brightness comparison 73-78.9 79-82.9 83-84.9 85-87.9 88+ 81.9 and below 82-86.9 87-90.9 91 +NO.1 NO.2 NO.3 NO.4 GE BRIGHTNESS METER Premium Old New 56
57. 57. Pele Oy How to improve brightness and opacity? 57 Action s k brightness opacity Bleaching –– ++ – Mechanical pulp refining + + Chemical pulp refining –– – – Adding carbonate filler ++ – + ++ Adding OBA/FWA – + Adding blue dye + – + Adding other dyes ++ –– ++ Wet pressing – – – Drying paper more + + + Surface sizing – – – Calendering – – –
58. 58. Pele Oy 58
59. 59. Pele Oy Color blindness test 1 59 5 5 8 9 What are the numbers inside circles?
60. 60. Pele Oy 60 Color blindness test 2 3 10 5 9 What are the numbers inside circles?